This invention pertains to RF (radio frequency) transmission circuitry and deals more particularly with the use of a unique feedback circuit to permit low level modulation of an ordinary radio transmitter.
Unwanted variation in modulation and power output levels in radio transmitters prompted by variations in supply voltage, amplifier gain and distortion has been a long-standing problem in the art. Also, in the past, in order to achieve the desired signal transmission range, it was sometimes necessary to approach one hundred percent modulation of the transmitter; however, this mode of operation is generally undesirable since distortion is usually increased and occasional over-modulation caused by such distortion results in the transmitted signal being "splattered" over from the allocated band to other broadcasting bands in violation of federal regulatory standards. Consequently, modulation levels lower than one hundred percent must generally be used to avoid splattering; however, prior art modulation circuit designs share the common disadvantage that the radiated power associated with the up-peak of the modulated wave is generally less than the power associated with the down-peak thereof when operating at less than 100 percent modulation. This imbalance between up-and-down peaks of the modulated wave reduces the effective transmission range of the transmitter. A still further problem to be dealt with in the design of radio transmitter circuits is the tendency of the RF amplifier to self-oscillate, overdrive itself, or even burnout when abnormally high waves are reflected back from the antenna toward the transmitter.
The use of feedback circuits in the first stages of a radio transmitter and particularly AM (amplitude modulation) transmitters, for controlling the level of RF power output is an art recognized concept. For example, U.S. Pat. No. 3,808,539 to Martin, discloses a feedback circuit loop connecting selected stages of an RF amplifier for providing a direct current feedback voltage to an earlier stage, when the RF output power of the amplifier diminishes as a result of battery drain or the like, in order to increase the gain of the earlier stage and therby maintain the RF power output of the transmitter more nearly constant. The feedback circuit disclosed by Martin however, in no way controls modulation as contemplated by the present invention. U.S. Pat. No. 2,085,125 to Shaw, teaches the use of a pair of feedback circuits coupled between a receiver for monitoring the transmitted radio waves, and the audio and RF stages respectively of the transmitter. The approach by Shaw requires the use of a separate receiving means including a reception antenna physically spaced from the transmitting antenna, and is therefore impractical for use by a mobile transmitter such as a hand-held transceiver. Moreover, neither of the feedback circuits of Shaw are employed to achieve low level modulation of the transmitter. In any event, none of the circuits taught by these prior art patents is operable to compensate for the effects of abnormally high levels of reflected waves which may exist on the antenna line.
The present invention overcomes deficiencies inherent in prior art designs by providing especially simple feedback circuit means integral with the transmitter which not only maintains a constant level of RF output power, but compensates for the effects of waves reflected on the antenna line, reduces noise and distortion in the modulating portion of the transmitted radio wave, and provides modulation of the transmitter using low level modulation signals and a single feedback loop. According to the present invention, a single feedback circuit coupled between the initial stage of a multistage type RF amplifier and the transmitter antenna line includes a directional coupling device and envelope detection circuitry for deriving signal information from the modulated carrier wave just prior to transmission of the latter. The signal information is combined with the output of a modulating signal source to form a conditioned, low level modulation signal which is processed by an operational amplifier and is delivered through a shunting diode to the collector of the initial stage of a multistage, transistorized RF amplifier. The shunting diode is responsive to the composite waveform characteristics of the conditioned modulating signal to selectively drain RF energy from the initial amplifier stage in order to maintain the amplifier output level nearly constant and to protect against overdriving of the amplifier in the event that high levels of reflected waves are present on the antenna line. One or more potentiometers is coupled between the feedback circuit and a direct current voltage source to conveniently set the quiescent operating point of the feedback circuit, which in turn predetermines the RF output level and modulation percentage of the transmitter.
The primary object of the invention is to provide a unique method of, and circuit means for, low level signal modulation of a radio frequency transmitter.
Another object of the invention is to provide unique circuit means for maintaining the power output of a radio frequency transmitter at a relatively constant level in spite of variations in power supply level, degradation of electrical components over time, amplifier distortion, changes in amplifier gain, and reflected waves present on the antenna line.
A further object of the invention is to provide unique circuit means of the type described which yields full up-peak modulation of a carrier wave in a radio frequency transmitter even in the presence of voltage standing waves on the antenna line, thereby increasing the power output and transmission range of the transmitter.
Still another object of the invention is to provide a unique method of, and circuit means for, amplitude modulation of a radio frequency carrier wave without the need for an audio frequency transformer device. As a corollary to the foregoing object, it is a further aim of the invention to modulate an early stage of a multistage RF amplifier using a low level modulation signal.
Another object of the invention is to provide a unique means for extracting a modulated RF carrier signal, prior to transmission of the latter, for purposes of feeding back certain information components of the modulated wave in a feedback circuit in order to control RF power output, audio distortion and modulation percentage of a radio transmitter.
A further object of the invention is to provide an especially simple means for adjusting the RF power output of a radio transmitter using a single potentiometer device.
Other and further objects of the invention will be made clear or become apparent in the course of the following description.